[json] Add file extension association for JSONC files

[StephenWeatherford]

• VSCode Version: 1.23.0-insider
• OS Version: win64

Steps to Reproduce:

1. Open a JSONC file (JSON with comments)
   !) The file is opened in "Plain Text" mode
   EXPECTED: It should be opened in JSONC mode

I am aware that you can change this via the status bar, but the change will not remain once you close the file. I'm also aware you can set this up in your user options (#47418 - thanks, by the way). But it seems like this should be a default setting set up by VS Code rather than having to have everyone know to do it manually. Thanks.

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Global User snippets not working for jsonc language id (#42539)